The present invention relates to screws for use in soft material such as plastic. The invention also has application in material such as cast aluminum and other soft metals.
Fastening to such material has been a problem particularly when a boss is formed in the workpiece to conserve material, to provide spacing, or to provide more material for engagement. Bosses tend to break as a result of radial stresses induced upon insertion of a screw. Frequently, when bosses are used, the boss location is the only suitable place for fastening. If the boss is damaged, no alternative exists but to discard an entire part.
Another problem associated with fastening to soft material is due to the fact that screws are often driven with mechanical drivers having settable torque limiting clutches. The use of such drivers makes it desirable to design screws with a large operating range (O.R). The O.R is the difference between the maximum amount of torque required to drive a screw and the minimum torque at which a screw will strip out. If a screw has a small O.R., variations (even within allowable tolerances) in hole size, screw size and finish, etc. will make proper setting of an automatic clutch difficult, because those variations will tend to make some screws undrivable and others strip out. Screws having a large O.R. make an effective setting of the clutch easier, because tolerance variations in the screw or workpiece will less often result in a screw falling outside the O.R. The result is that fewer screws are undrivable, fewer screws strip, and fewer parts need to be scrapped.
An object of the invention is to provide a screw having a thread which tends to increase strip torque without significant increases in radial stress.
Another object is to provide a screw thread form which increases strip torque without a proportionate increase in drive torque.
Another object is to provide a screw which has the above characteristics and which optimizes the use of blank material to allow for larger shank diameter for a given crest diameter.
These and other objects and advantages are achieved by providing a screw with a thread which has an upper flank with a compound surface which tends to increase the frictional engagement between the screw and the workpiece only after the head of the screw contacts the article being fastened. Tightening of the screw causes workpiece material to deform into frictional contact with the upper flank, and the upper flank is shaped to optimize friction while advantageously distributing radial stress.
The objects and advantages of the present invention will be better understood by reading the following specification in conjunction with the accompanying drawings.